As cell phones with cameras in which a camera is mounted have become popular in recent years, chances photographing various objects to be photographed by using the cell phone have increased. For example, there is a case that an object such as a friend or landscape which is apart from a lens of a camera to some extent is photographed (normal photographing) and, alternatively, there is a case that an object such as a timetable for bus or petals of a flower at a position near the lens of the camera is photographed (close-up photographing).
In the close-up photographing (macro-photographing), a lens position of a camera is required to locate at a position nearer on an object side than a lens position at the time of normal photographing. Therefore, this type of photographing lens system is provided with a drive mechanism for driving a lens to move in the optical axis direction. The drive mechanism is driven by changing a switch to move a lens in the optical axis direction (see, for example, Japanese Patent Laid-Open No. 2005-128392, Japanese Patent Laid-Open No. 2005-165058 and Japanese Patent Laid-Open No. 2007-148354).
A cell phone with camera or the like is often put in a pocket or a bag and carried and thus a large shake or impact may be applied to the cell phone at the time of carrying (when it is not used). In other words, in a state where a cell phone is not used, a drive mechanism for moving a lens is not operated and thus the lens is easily vibrated due to a shake or an impact. As a result, the lens is easily displaced or damaged by the shake or the impact.
In order to prevent this problem, the lens drive device which is disclosed in the above-mentioned former two Patent References include a yoke, a base, a magnet and a coil, a lens support body which supports a lens, a front side spring (object side spring), a rear side spring (image sensor element side spring) and another base. When energization to the coil is stopped, the front side spring and the rear side spring apply urging forces to the lens support body so that the lens support body is pressed against the base. In this manner, even when a large shake or impact occurs at the time of carrying, shaking and backlash are less likely to occur in the lens support body to provide the lens drive device with a shock resistant property.
Further, the lens drive device disclosed in the last Patent Reference includes a movable lens body, a fixed body which movably supports the movable lens body in an optical axis direction of a lens, a magnetic drive mechanism for moving the movable lens body in the optical axis direction and a holder support. Further, a coil is provided in the movable lens body and a magnet is provided in the fixed body as the magnetic drive mechanism. In addition, a magnetic spring (magnetic member) is provided in the movable lens body.
The magnetic spring is magnetically attracted by the magnet which is provided in the fixed body. In this manner, when energization to the coil is stopped (not-energized time), the movable lens body is abutted with and pressed down to the fixed body by the magnetic attractive force so that the movable lens body does not wobble.
More specifically described with reference to half sectional schematic views in FIGS. 4(a) through 4(d). FIGS. 4(a) through 4(d) are cross sectional schematic views showing conventional lens drive devices. FIGS. 4(a) and 4(b) are schematic views showing a lens drive device 100 disclosed in the above-mentioned former two Patent References. Further, FIGS. 4(c) and 4(d) are schematic views showing a lens drive device 100 disclosed in the last Patent Reference.
As shown in FIGS. 4(a) and 4(b), the conventional lens drive device 100 is structured of a sleeve 101, a yoke 102, a cover 110 which is provided on an object side in an optical axis direction so as to interpose the yoke 102, a base portion 111 which is provided on an image sensor element side, a coil 103, a magnet 104, plate springs 105, abutting parts 107 and 108 for restricting a moving amount in the optical axis direction of the sleeve 101. Further, in FIGS. 4(c) and 4(d), in addition to the above-mentioned structure, a magnetic member 106 for structuring a magnetic spring is disposed on an upper end face in the optical axis direction of the sleeve 101 and on an under side of the upper plate spring 105.
An operation of the lens drive device 100 will be described below with reference to FIGS. 4(a) through 4(d). FIG. 4(a) is a view showing a state where the coil is not energized, i.e., at a home position, and FIG. 4(b) is a view showing a state where the coil is energized, i.e., a state where the sleeve 101 has been moved on the object side. Similarly, FIG. 4(c) is a view showing a state at a home position and FIG. 4(d) is a view showing a state where the sleeve 101 has been moved on the object side.
In the lens drive device 100 shown in FIG. 4(a), when the coil 103 is not energized, the abutting part 107 of the sleeve 101 is pressed to be abutted with the base 111 by urging forces of the plate springs 105 and thus the sleeve 101 is located at a non-energized position. Further, when the coil 103 is energized, the sleeve 101 is moved in the optical axis direction to a position shown in FIG. 4(b). In this case, a driving force (thrust force) by the coil 103 and the urging forces by the plate springs 105, i.e., an elastic force for restricting movement of the sleeve 101 act on the sleeve 101. Therefore, the sleeve 101 is stopped at a position where these forces are balanced, i.e., at an energized position.
On the other hand, in the lens drive device 100 shown in FIG. 4(c), the magnetic member 106 for structuring a magnetic spring together with the magnet 104 is magnetically attracted by the magnet 104 and the sleeve 101 is pressed down and abutted with the base 11 by the magnetic attractive force when the coil 103 is not energized and thus the sleeve 101 is located at a non-energized position. The force Fp with which the magnetic member 106 urges the sleeve 101 is set larger than the weight Wp of the sleeve 101. In other words, in this non-energized state in the lens drive device 100, the plate springs 105 hardly apply their elastic forces to the sleeve 101. On the other hand, when the coil 103 is energized, the sleeve 101 is moved in the optical axis direction to a position shown in FIG. 4(d). In this case, a driving force of the coil 103 and the urging forces (elastic force) of the plate springs 105 act on the sleeve 101, and the sleeve 101 is stopped at a position where these forces are balanced with each other and thus the sleeve 101 is located at an energized position. In this lens drive device 100, a magnetic attractive force between the magnetic member 106 and the magnet 104 is smaller than the driving force of the coil 103 but affects movement of the sleeve 101.
However, in the lens drive device 100 shown in FIG. 4(a), when the coil 103 is not energized (home position), the abutting part 107 of the sleeve 101 is pressed to be abutted with the base 111 by the elastic forces (urging force) of the plate springs 105 and thus the sleeve 101 is located at the non-energized position. Therefore, the plate springs 105 are always resiliently bent both at the not-energized time and at the energized time of the coil 103. In addition, in the state where the sleeve 101 is moved to the object side shown in FIG. 4(b), the plate springs 105 which have been already resiliently bent at the home position as shown in FIG. 4(a) are required to be resiliently bent further more. Therefore, the metal spring may occur metal fatigue or permanent deformation. In addition, when the plate spring 105 has been continuously used at the spring deflection limit value or its vicinity of metal material of the plate spring 105, fatigue failure is likely to occur to the plate spring 105.
Especially, according to miniaturization demand for an actuator in recent years, when the plate spring 105 is made further smaller or thinner, the strength of the plate spring 105 becomes weaker or the plate spring 105 becomes easily to bend and thus fatigue failure may occur.
On the other hand, in the lens drive device 100 as shown in FIG. 4(c), when the coil 103 is not energized, the sleeve 101 is pressed to be abutted with the base 111 by using the magnetic attractive force between the magnetic member 106 and the magnet 104 and thus, as shown in FIG. 4(c), the plate springs 105 hardly bend resiliently at the home position (non-energized position). However, when the size of the lens drive device 100 is reduced with miniaturization demand for an actuator, the magnetic member 106 becomes located at a nearer position to the magnet 104. As a result, the magnetic attractive force of the magnet 104 to the magnetic member 106 becomes larger. Especially, the magnetic attractive force in the radial direction becomes larger than that in the thrust direction. Therefore, assembling accuracy is required to enhance so that the sleeve 101 and the magnet 104 are concentrically disposed with respect to the optical axis and thus workability is deteriorated. In other words, when the assembling accuracy is not satisfactory, after the sleeve 101 and the magnet 104 have been assembled into the lens drive device 100, the sleeve 101 may be inclined by the effect of the magnetic attractive force in the radial direction and, as a result, its optical characteristic is deteriorated.